1. Field of the Invention
The present invention relates to a pigment dispersing agent that may be used to stably disperse an inorganic or organic pigment in a nonaqueous organic medium at high concentrations and also a offset printing ink composition employing the same. More particularly, the present invention pertains to a pigment dispersing agent which is capable of exhibiting excellent printing performance when used in an offset printing ink.
2. Description of the Prior Art
In general, coating agents such as printing inks and paints are prepared by dispersing a finely divided pigment serving as a coloring agent in a medium called a vehicle which consists essentially of a binder resin and a solvent.
The pigment dispersion thus obtained is required to be stable without causing reaggregation of the pigment even after being stored for a long period of time and to be capable of forming a coating film which has excellent gloss and superior coloring properties.
In many cases, however, pigment dispersions prepared by dispersing finely divided pigments in vehicles have high viscosities, so that they are difficult to handle in such operations as removal from a dispersion mixer, stirring, transportation, etc. Therefore, the pigment concentration in the dispersions cannot unrestrictedly be increased from the viewpoint of operating efficiency.
For this reason, there is a demand for pigments that can be stably dispersed in vehicles at high concentrations and that permit the resulting dispersions to have a low level of viscosity.
The above-described demands depend upon the pigment dispersing performance of the vehicle employed and it is therefore a problem of considerable importance to develop vehicles which enable pigments to be effectively dispersed therein.
To this end, many studies have heretofore been conducted to improve binder resins that constitute vehicles. However, since binder resins themselves are polymers with high molecular weight and must maintain the important function of being film forming agents, the pigment dispersing performance thereof cannot be improved by a large margin. It is therefore actual practice to jointly use auxiliaries for dispersing pigments.
These dispersing auxiliaries are, however, a group of substances which are called surface active agents, and many of them have relatively low molecular weights. When such surface active agents are employed as dispersing auxiliaries for nonaqueous disperse systems, electrostatic repulsion cannot be expected to stabilize the dispersion and the stabilization of the dispersion must rely solely on the steric hindrance by the dispersing auxiliaries adsorbed on the surfaces of pigment particles. Accordingly, dispersing agents having relatively low molecular weights are not effective in providing satisfactory dispersion stability and present various restrictions with respect to applicability.
For instance, if a surface active agent having a relatively low molecular weight is used as a dispersing agent for an ink used in offset printing which is an image forming method utilizing the interfacial chemical repulsion that takes place between the water component, i.e., dampening water, and the oil component, i.e., oil-based ink, the emulsifying characteristics of the ink with respect to the dampening water are adversely affected, resulting in various problems such as scumming and greasing which are caused by the transfer of ink to the non-image area.
Various kinds of dispersing agent with polymer chains which have relatively high molecular weights have recently been proposed for the purpose of ameliorating the disadvantages of the conventional dispersing agents having relatively low molecular weights.
For example, Japanese Patent Publication No. 54(1079)-34009 (corresponding to BP 1,342,746) proposes a polyester oligomer obtained by esterification condensation of a hydroxycarboxylic acid, while Japanese Patent Publication No. 57(1982)-25251 (corresponding to BP 1,373,660) proposes a compound in the form of an amide obtained by carrying out reaction of the above-described polyester oligomer with an amine compound.
Further, U.S. Pat. No. 4,224,212 discloses a dispersing agent comprising a polyester oligomer and a polyalkylene imine having molecular weight, more than 500, preferably 10,000-100,000, which are linked together.
Japanese Patent Kokai (Laid-Open) No. 61(1986)-163977 discloses a reaction product of a polyester oligomer with either piperazine or alkyl piperazine.
The proposed dispersing agents are, however, still unsatisfactory in terms of their achievability to high pigment concentration and dispersion stabilization in nonaqueous disperse systems, although they are fairly effective in comparison with the conventional low-molecular dispersing agents. When these dispersing agents are used in offset printing inks, the emulsifying characteristics of the inks are adversely affected to a significant extent, so that satisfactory print results cannot be obtained. Thus, the prior art dispersing agents are impractical.